It's very interested dusscusion here - so I must confess that I personally known Master owner - and Piotr is really "crazy man" . All details of his products must be really "reality in miniature". I never chad a chance to make correct MG barrels myself - especially from PE sheet - I never was enough skilled modeller to doing this... So turned brass barrels was a real help for me.
One picture that's speak more about accuracy of MASTER barrels - You can see the comparision with the plans of the real thing here:
If You want to find more info about